Could Late-Night Exposure to Light Be Making You Fat?

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If you stay up late watching TV or bathed in the glow of a computer screen, you probably aren’t doing your waistline any favors. A new study finds that exposure to light at night may contribute to weight gain, independent of diet or exercise levels.

For the study, released Monday in the online edition of the Proceedings of the National Academy of Sciences, researchers at Ohio State University studied three groups of mice. Each was exposed to a different light cycle for eight weeks: one group was exposed to the standard 16 hours of bright light and 8 hours of dark; a second group got 24 hours of light; and a third experienced 16 hours of bright light, and 8 hours of dim light. (More on Time.com: The ‘Other’ Salt: 5 Foods Rich in Potassium)

All the mice ate the same amount of food and did the same amount exercise, but the mice that were exposed to even dim light at night gained 50% more weight than mice living in a natural light cycle. Researchers speculate that the absence of darkness may have tinkered with the animals’ biological clocks and their hormonal signals for feeding.

While mice in normal light-dark conditions ate about two-thirds of their chow during normal active hours (in this case, at night, since mice are nocturnal), those that had extra light ate more than half their food during periods normally meant for rest — not the optimal time for a body to be burning calories.

The findings suggest that it’s not only what you eat, but when you eat that affects weight. CNN’s Chart Blog reports:

Explaining how this might apply to humans, [lead author Laura] Fonken said that the dim-light mouse group could be compared to human exposure to television or computer time and eating during screen time — rather than during a meal time — is not uncommon, which might explain human weight gain. Previous studies have shown higher body mass indexes and increased rates of cardiovascular disease in overnight shift workers, she said, another instance where eating during traditional rest periods may apply and add to weight gain.

In a second set of experiments, researchers again studied the effect of light cycles on mice, but this time controlled when animals ate. Mice exposed to dim nighttime light again gained more weight than normal, except when their mealtimes were restricted to normal activity hours. Dim-light mice that ate when they were supposed to maintained the same weight as mice experiencing normal light-dark cycles. (More on Time.com: Want Good Health? There Are 10 Apps for That)

While lighter nights may trigger shifts in the biological clock, previous research suggests eating at odd hours may do the same — creating a cycle that compounds itself. An article in Science News suggests more study is needed before drawing any solid conclusions in humans:

The study by [co-author Randy] Nelson’s group “is solid” and consistent with studies by others showing that nighttime eating by people can foster weight gain and prediabetic changes in glucose control, notes Satchidananda Panda of the Salk Institute for Biological Sciences in La Jolla, Calif. “Unfortunately,” he adds, “there is not even a single study in humans looking at temporal spreading of caloric intake” — looking to see if fat deposition or glucose control is affected by when a given set of calories is consumed.

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